The present invention relates generally to integrated circuit memories, and more specifically to a method and structure for recovering smaller densities memories from larger density memories.
Often, smaller density memory devices are packaged in the same package type, having the same number of pins and other physical dimensions, as larger density memory devices. This is true for many memory devices, including SRAMs (Static Random Access Memories), FIFO (First In First Out) memories, and tagRAMs, etc. For example, a 64K (8K.times.8) SRAM is typically packaged in the same package as a 256K (32K.times.8) SRAM. Also, the 1 Meg Burst RAM (64K.times.18), a data cache SRAM memory produced by SGS-Thomson Microelectronics, Inc., is packaged in the same package as the 512K Burst RAM (32K.times.18).
Currently manufacturers of integrated circuit memory devices do not have a satisfactory method for recovering the yield fallout of memory devices as smaller density memory devices in the same package. For, although a larger density memory device and a smaller density memory device may be housed in an identical package, bonding for the two memory devices is typically quite different. As an example, consider a 256K SRAM and a 64K SRAM. Typically, both the 256K and 64K SRAM are packaged in a 28 pin DIP or SOJ package as illustrated in FIGS. 1a and 2a, respectively. The pin names of the 28 pin package of FIGS. 1a and 2a are shown in FIGS. 1b and 2b, respectively. The 64K SRAM of FIGS. 2a and 2b has 13 address pins, or two less than the 15 address pins of the 256K SRAM of FIGS. 1a and 1b. According to FIGS. 2a and 2b, the 64K SRAM differs from the 256K SRAM because pin 1 is a No Connection (NC) pin rather than an address pin and pin 26 is a Chip Enable (E2) pin rather than an address pin. Because pins 1 and 26 of the 64K and 256K SRAMs are used for different purposes, they have different bonding connections. Otherwise, all other pins of the two devices have identical functions and thus have the same bonding connections.
Partially due to the fact that the 64K and the 256K SRAMs have different bonding connections, to date, the yield fallout of 256K SRAMs are not easily recoverable as fully functional 64K SRAMs in spite of the fact the two devices are housed in identical packaging. Additionally, even if it is possible to salvage yield fallout, sorting problems encountered with sorting the memory devices as either 256K SRAMs or 64K SRAMs may make it not practical to do so. As a result, yield fallout from larger density memory devices such as the 256K SRAM is typically scrapped rather than recovered as a fully recoverable smaller density device.
One possible solution that has been examined as a way to recover yield fallout of a larger density memory device as a small density memory device involves a double inking technique. According to the double inking technique, typically, those devices which could be recoverable as a larger density memory device are not marked, those devices which are not recoverable are marked with one ink spot, and those devices which could be recoverable as a smaller density device are marked with two ink spots during wafer testing. Thus, those devices of a 256K SRAM wafer which would be recoverable as a 64K SRAM device might be differentiated from other devices by two ink marks, rather than one or no ink marks. Then at the device assembly step, these double inked devices would be separated from the 256K devices of the wafer and bonded differently according to the bonding requirements of a smaller density 64K SRAM.
A concern with the double inking technique is that it involves many backend steps, is more expensive, and is more time consuming than traditional device handling and is subject to errors. The double inking technique is complicated further when a semiconductor manufacturer uses multiple assembly subcontractors and this, of course, complicates the manufacturing process and drives up the cost of the device.